Q-switching is a common and effective technique to achieve optical pulses with short duration, high repetition rate and high peak power. These characteristics are required for laser ranging, nonlinear studies, medicine, laser micro-machining, and other important applications. Q-switching can be effected using an active device which is controlled or driven by an external signal. Q-switching can also be performed using a passive structure that has no external control, but instead operates periodically as a result of its own properties. The present invention relates to a laser system using such a passive Q-switching method.
Passive Q-switching employing a saturable absorber as a Q-switch element is economical, simple and practical. There are many different materials and configurations for passive Q-switching.
U.S. Pat. No. 3,997,854 entitled “Passive Q-Switch Cell”, issued to Buchman et al, discloses a passive Q-switch cell of a liquid saturable solution of dye used on the laser wavelength of 1.06 μm. U.S. Pat. No. 4,637,030 entitled “Switching Laser”, issued to Midavaine et al, discloses a passive Q-switching method with absorption gas used on the laser wavelength of 10.6 μm. U.S. Pat. No. 5,414,724 entitled “Monolithic Self Q-Switched Laser”, issued to Zhou et al, discloses a monolithic self-Q-switched laser with a single Cr:Nd:YAG crystal to generate laser pulses at the wavelength of 1.06 μm. U.S. Pat. No. 5,832,008 entitled “Eyesafe Laser System Using Transition Metal-Doped Group II-VI Semiconductor as a Passive Saturable Absorber Q-Switch”, issued to Bimbaum et al, discloses a passive Q-switch element of Co:ZnSe for laser system at the wavelength of 1.54 μm and 1.6 μm. U.S. Pat. No. 5,724,372 entitled “Diode-Pumped Laser System Using Uranium-Doped Q-Switch”, issued to Stultz et al, discloses a diode-pumped Er:Yb:Glass laser with an output from about 1.5 μm to 1.6 μm and Q-switched with U:CaF2. U.S. Pat. No. 5,802,083 entitled “Saturable Absorber Q-Switches for 2-μm Laser”, issued to Bimbaum et al, discloses a passively Q-switched laser with an output from about 1.6 μm to 2.3 μm using Ho:YLF or Ho:YVO4 as Q-switch material. U.S. Pat. No. 5,237,577 entitled “Monolithically Integrated Fabry-Perot Saturable Absorber”, issued to Keller et al, discloses a Fabry-Perot saturable absorber with a construction of multiple quantum well of AlAs/GaAs, which can be used as a saturable absorber in passive Q-switching and passive modelocking. It was also used as the end mirror of a diode-end-pumped Nd:YLF laser.
Active Q-switching allows a user to vary the output characteristics of a laser beam. A major disadvantage of passive Q-switching, as compared to active Q-switching, is non-adjustability of the parameters of the Q-switched pulses. There is no effective way to control or adjust the parameters of passive Q-switching. It would be useful to devise a technique that is capable of adjusting the parameters of passively Q-switched pulses using a single saturable absorber in the laser cavity.
Saturable absorbers are known to be key elements for passive Q-switching. Materials such as solids, liquids and gases have been used as saturable absorbers based upon the chosen wavelength of laser operation. Generally, the theoretically shortest pulse duration achievable from a Q-switched laser system is limited by the round-trip time of the laser cavity. The shorter the laser cavity, the shorter the Q-switched pulse duration. Therefore shortening the laser cavity is an effective way to get shorter pulses.
Existing Q-switches are mainly made of active devices that make use of acousto-optic or electro-optic effects. These Q-switches require drivers to operate. It would be desirable to provide a tunable passive Q-switch that is a simple device requiring no external drivers and hence may offer reduced operating costs and can render a laser resonator very compact.
Kajava et al [Optics Letters, 21(6), 1996] use a piece of GaAs wafer inside the cavity of an end-pumped Nd:YAG laser to achieve passive Q-switching. The GaAs wafer has a fixed transmittance value. By changing the pump power, the output characteristics of the laser, such as the pulse repetition rate and pulse duration, will vary. However, there are disadvantages in using pump power as the control variable as the emission wavelength of the diode laser may drift away from its optimum value, the temperature of the diode laser may take a long time to stabilise, and the beam quality of the Nd:YAG laser output may change substantially.
Through experimental and theoretical studies [J. H. Gu, et al, SPIE Vol. 3398, pp.170-177, November 1999], it is found that the GaAs wafers used for passive Q-switching may exhibit Fabry-Perot (F-P) effect. Furthermore, effective transmittance of a GaAs wafer, which is generated by this F-P effect, could create a large impact on the output characteristics of the passively Q-switched laser. Based on this prior art, further improvements were made and the following details the improvements.